I'm not going to get further into the "safety" argument, I've been on the forum for a while and it's not like we're going to come to some huge agreement.

The things that nearly everyone agrees on are that -
1. No doubt it's safer to have a rear blinky that none
2. Two rear blinkies are better than one because you have a backup in case one stops working - and because if one does stop working you don't know because the light is behind you
3. You're more visible as a bike if at least one of them is blinking.

The only other thing I know for sure is that as a driver, I prefer bikes with the 1 blinking + 1 steady approach. Second is a blinking light. I don't like a purely steady light as it's easy to get lost among a lot of lights, and obviously I really don't like no lights at all. I feel like steady + blinking is the right blend between visible and obnoxious, when I'm driving. However there are no absolutes - when driving through downtown everything is so lit up tons of blinking doesn't really bother me either. I also might feel differently if I was biking alongside high speed unlit roads, but I generally just don't have to bike along them.

For whatever reason, I also find rear lights on the backs of helmets to be the kind of thing I don't notice until I get really close, regardless of whether they're blinking or not. I'm not entirely sure why that is, except that I've had it happen again and again - maybe it's up higher and I'm not scanning for lights at the level, or maybe it's because they're never as bright as tail lights. The only thing I would definitely say is don't use only a helmet mounted rear light, put at least one light around seat / car headlight level.